In Layer 0 optical networks, the time required to restore traffic channels is primarily dominated by the channel add time on the restoration path. The channel additions onto a lit optical path that already contains in-service channels are traditionally slow. This is due to the fact that, (1) for each Optical Multiplex Section (OMS), the new capacity addition needs to be controlled in order to minimize any power or Optical Signal to Noise Ratio (OSNR) impact on the already in-service channels on that section, and (2) the channel add operation from one OMS to the next needs to be sequential or pseudo-sequential in nature. The OMS is an all-optical section of an optical network between Optical Add/Drop Multiplexers (OADM), and each OMS has the property of having the same number of ingress and egress channels. The OMS may also be referred to as an optical section.
While adding in an OMS, two factors require attention: (1) the fast-transient impact on in-service channels' power and OSNR due to control loops of optical amplifiers, and (2) the steady-state power offsets (overshoots or undershoots from their optimal launch power targets to the fibers), and hence, OSNR impact on the in-service channels due to non-linear gain transfers over the photonic section due to Stimulated Raman Scattering (SRS) in optical fibers and Spectral Hole Burning (SHB) in Erbium-Doped Fiber Amplifiers (EDFAs). The fast-transient impact can be mostly minimized by introducing new channels on the path at a slow rate of 100's of milliseconds. To minimize the steady-state power offset, new channels are introduced in a controlled manner in several incremental power-add steps or in channel bundles, and between each incremental step or bundle, control algorithms are run to change the per channel actuators or amplifier gains to eliminate the incurred offset. Most of the time spent on adding channels in an OMS are to minimize the steady-state offset on in-service channels that typically takes seconds to tens of seconds.
As mentioned above, the channel add in downstream OMSs are typically either sequential or pseudo-sequential with respect to the channel add taking place in upstream OMSs. This is because the downstream OMS must wait for steady per channel input power coming from upstream prior to setting actuators to avoid setting incorrect target attenuations and gain. Hence, a capacity change operation through a cascade of OMSs is sequential, and in some cases, it is possible to reduce this to a pseudo-sequential operation using adaptive controller settings. If the attenuation targets in each OMS are preset to try to achieve a fast capacity change or restoration without considering the nonlinear gain transfers, it is common that the attenuation targets in each OMS would be very incorrect leading to non-optimal performance, which can then take a significant amount of time to reoptimize cascaded sections end-to-end leading to instability. Such large inaccuracy in attenuation settings at each OMS section leads to overshoots or undershoots to launch powers into the fiber that not only impacts the Signal to Noise Ratio (SNR) of the newly adding channels, but also creates the risk of impacting the available SNR margin of the in-service channels (e.g., by stealing gain through SRS thereby, reducing power and OSNR, or by giving too much power and introducing Self Phase Modulation (SPM) penalties, or even through channel-to-channel nonlinearities such as Cross Phase Modulation (XPM) and Four Wave Mixing (FWM) in the case of higher than expected neighboring channel powers). Therefore, channel additions over an optical path are primarily sequential, and Reconfigurable Optical Add/Drop Multiplexer (ROADM) hop dependent.
The issue of significant non-linear gain transfer over a capacity change is largely minimized when a system operates close to a full-fill spectral loading condition, and one option is always to load the system using “dummy” channel holders typically composed of carved Amplified Spontaneous Emission (ASE) which can then be swapped for real channels at a later time. This approach requires additional networkwide hardware installation and hence, impact on capital expenditure in day one installation. Design consideration also needs to be adjusted for the hardware failure scenarios with channel holders, where the source of “dummy” channels can fail for an optical section, leading to channel additions back to the traditional way. In addition, the presence of ASE channel holders gives the worst-case performance margin in cases that are not fully filled, since, in this scenario, there are always full fill interferers that can significantly increase nonlinear impairments compared to a partially loaded system. This can prevent capacity mining of unused performance margin and operators tend not to want to operate at the worst-case performance margin when not required.
Accordingly, it would be advantageous to address the nonlinear gain transfer without using “dummy” channels to achieve path independent Layer 0 timing to add/delete/optimize over a capacity change.